Thermoelectric energy storage system and method for storing thermoelectric energy

ABSTRACT

A thermoelectric energy storage system includes an intercooler for intercooling a working fluid between two compression stages. The intercooling may be carried out by flashing a portion of the working fluid taken from the output of an expander in a flash intercooler and/or by heating a secondary thermal storage with a further heat exchanger.

RELATED APPLICATIONS

This application claims priority as a continuation application under 35U.S.C. §120 to PCT/EP2011/057799, which was filed as an InternationalApplication on May 13, 2011 designating the U.S., and which claimspriority to European Application 10164288.2 filed in Europe on May 28,2010. The entire contents of these applications are hereby incorporatedby reference in their entireties.

FIELD

The present disclosure relates generally to the storage of electricenergy. More particularly, the present disclosure relates to a systemand a method for storing electric energy in the form of thermal energyin a thermal energy storage.

BACKGROUND INFORMATION

Base load generators such as nuclear power plants and generators withstochastic, intermittent energy sources such as wind turbines and solarpanels, etc. generate excess electrical power during times of low powerdemand. Large-scale electrical energy storage systems are a means ofdiverting this excess energy to times of peak demand and balance theoverall electricity generation and consumption.

In EP-A 1577548, the concept of a thermoelectric energy storage (TEES)system is disclosed. A thermoelectric energy storage converts excesselectricity to heat in a charging cycle, stores the heat, and convertsthe heat back to electricity in a discharging cycle, when necessary.Such an energy storage system may be robust, compact, site independentand may be suited to the storage of electrical energy in large amounts.Thermal energy may be stored in the form of sensible heat via a changein temperature or in the form of latent heat via a change of phase, or acombination of both. The storage medium for the sensible heat may be asolid, liquid, or a gas. The storage medium for the latent heat occursvia a change of phase and may involve any of these phases or acombination of them in series or in parallel.

The round-trip efficiency of an electrical energy storage system may bedefined as the percentage of electrical energy that can be dischargedfrom the storage in comparison to the electrical energy used to chargethe storage, provided that the state of the energy storage system afterdischarging returns to its initial condition before charging of thestorage. Thus, in order to achieve high roundtrip efficiency, theefficiencies of both modes need to be maximized inasmuch as their mutualdependence allows.

The roundtrip efficiency of the thermoelectric energy storage system islimited for various reasons rooted in the second law of thermodynamics.The first reason relates to the coefficient of performance of thesystem. When the system is in the charging mode, its ideal efficiencymay be governed by the coefficient of performance (COP) of a heat pump.The COP depends on the temperatures of the cold side (Tc) and the hotside (Th) as given by

${COP} = \frac{T_{h}}{T_{h} - T_{c}}$

Thus, it can be seen that the COP of a heat pump declines with increaseddifference between input and output temperature levels. Secondly, theconversion of heat to mechanical work in a heat engine is limited by theCarnot efficiency. When the system is in the discharging mode, theefficiency (η) is given by

$\eta = \frac{T_{h} - T_{c}}{T_{h}}$

Thus, it can be seen that efficiency increases when the cold sidetemperature decreases.

Thirdly, any heat flow from a working fluid to a thermal storage andvice versa requires a temperature difference in order to happen. Thisfact inevitably degrades the temperature level and thus the capabilityof the heat to do work.

It is noted that many industrial processes involve the provision ofthermal energy and storage of the thermal energy. Examples arerefrigeration devices, heat pumps, air conditioning and the processindustry. In solar thermal power plants, heat is provided, possiblystored, and converted to electrical energy. However, all theseapplications are distinct from thermoelectric energy storage systemsbecause they are not concerned with heat for the exclusive purpose ofstoring electricity.

In EP-A 2157317, the concept of a transcritical thermoelectric energystorage is disclosed. In such a system, the working fluid undergoestranscritical cooling during the charging and transcritical heatingduring the discharging cycle as it exchanges heat with the thermalstorage medium.

U.S. Pat. No. 3,165,905 (Ware) describes a refrigerating machineincluding an economizer with the aim of improving the efficiency of therefrigerating cycle.

An article entitled “The Commercial Feasibility of the Use of WaterVapor As a Refrigerant” by Lachner B. F., Nellis G. F., Reindl D. T.(2007) International Journal of Refrigeration 30, 699-708, describes theuse of flash intercooling in between compression stages in order toimprove the coefficient of performance of refrigeration systems.

However, in certain cases, it would be disadvantageous to apply suchtechniques for improving the efficiency of refrigeration cycles to asystem having both charging and discharging cycles, since in applyingsuch techniques to such a system, an efficiency improvement in onecycles could result in an efficiency reduction in the other cycle.

SUMMARY

An exemplary embodiment of the present disclosure provides athermoelectric energy storage system for storing electrical energy bytransferring thermal energy to a thermal storage in a charging cycle,and for generating electricity by retrieving the thermal energy from thethermal storage in a discharging cycle. The exemplary thermoelectricenergy storage system includes a working fluid circuit configured tocirculate a working fluid, a first compressor configured to, in thecharging cycle, compress the working fluid from a low pressure to anintermediate pressure, and an intercooler configured to, in the chargingcycle, cool the working fluid at the intermediate pressure. Theexemplary thermoelectric energy storage system also includes a secondcompressor configured to, in the charging cycle, compress the workingfluid from the intermediate pressure to a high pressure, and a firstheat exchanger configured to, in the charging cycle, transfer heat fromthe working fluid at the high pressure to the thermal storage and, inthe discharging cycle, transfer heat from the thermal storage to theworking fluid at the high pressure.

An exemplary embodiment of the present disclosure provides a method forstoring electrical energy in a charging cycle and retrieving electricalenergy in a discharging cycle. In the charging cycle, the exemplarymethod includes compressing the working fluid from a low pressure to anintermediate pressure for storing electrical energy, cooling the workingfluid at the intermediate pressure, compressing the working fluid fromthe intermediate pressure to a high pressure for storing electricalenergy, and transferring heat from the working fluid at the highpressure to the thermal storage. In the discharging cycle, the exemplarymethod includes transferring heat from the thermal storage to theworking fluid at the high pressure, and expanding the working fluid fromthe high pressure for generating electrical energy.

BRIEF DESCRIPTION OF THE DRAWINGS

Additional refinements, advantages and features of the presentdisclosure are described in more detail below with reference toexemplary embodiments illustrated in the drawings, in which:

FIG. 1 a shows a simplified schematic diagram of a charging cycle of athermoelectric energy storage system according to an exemplaryembodiment of the present disclosure;

FIG. 1 b shows a simplified schematic diagram of a discharging cycle ofa thermoelectric energy storage system according to an exemplaryembodiment of the present disclosure;

FIG. 2 a shows an enthalpy-pressure diagram of the heat transfer in thecharging cycle of a transcritical thermoelectric energy storage systemaccording to an exemplary embodiment of the present disclosure;

FIG. 2 b shows an enthalpy-pressure diagram of the heat transfer in thedischarging cycle of a transcritical thermoelectric energy storagesystem according to an exemplary embodiment of the present disclosure;

FIG. 3 a shows an enthalpy-pressure diagram of the heat transfer in thecharging cycle of a thermoelectric energy storage system according to anexemplary embodiment of the present disclosure; and

FIG. 3 b shows an enthalpy-pressure diagram of the heat transfer in thedischarging cycle of a thermoelectric energy storage system according toan exemplary embodiment of the present disclosure.

For consistency, in general, the same reference numerals are used todenote identical or similarly functioning elements illustratedthroughout the drawings.

DETAILED DESCRIPTION

Exemplary embodiments of the present disclosure provide an efficientthermoelectric energy storage having a high round-trip efficiency, whileminimizing the system costs involved.

Exemplary embodiments of the present disclosure provide a thermoelectricenergy storage system and method for storing electrical energy in acharging cycle and retrieving electrical energy in a discharging cycle.

In accordance with an exemplary embodiment of the present disclosure,the thermoelectric energy storage system stores electrical energy bytransferring thermal energy to a thermal storage in a charging cycle,and generates electricity by retrieving the thermal energy from thethermal storage in a discharging cycle.

According to an exemplary embodiment of the present disclosure, thethermoelectric energy storage system includes a working fluid circuitconfigured to circulate a working fluid, a first compressor configuredto, in the charging cycle, compress the working fluid from a lowpressure to an intermediate pressure (such that the temperature of theworking fluid is rising), and an intercooler configured to, in thecharging cycle, cool the working fluid at the intermediate pressure (forlowering the temperature of the working fluid). In addition, theexemplary thermoelectric energy storage system includes a secondcompressor configured to, in the charging cycle, compress the workingfluid from the intermediate pressure to a high pressure, a first heatexchanger configured to, in the charging cycle, transfer heat from theworking fluid at the high pressure to the thermal storage and, in thedischarging cycle, transfer heat from the thermal storage to the workingfluid at the high pressure.

According to an exemplary embodiment of the present disclosure, theworking fluid may be compressed in two stages: from the low pressure tothe intermediate pressure in a first stage and from the intermediatepressure to the high pressure in a second stage.

According to an exemplary embodiment of the present disclosure, theintercooler includes a flash intercooler and/or a second heat exchanger.In other words, the intercooling may be carried out by (a) flashing aportion of the working fluid (taken from the output of a expander) inthe flash intercooler and/or by (b) heating a secondary thermal storagewith the second heat exchanger. This may have the advantage of (a)reducing the compressor energy of the first stage without compromisingthe thermal energy delivered to the main thermal storage and/or of (b)carrying out a reheat in the discharging cycle by using the secondarythermal storage to increase the power output.

This may mean that second heat exchanger, in the charging cycle,transfers heat from the working fluid at the intermediate pressure to asecond thermal storage and, in the discharging cycle, transfers heatfrom the second thermal storage to the working fluid at intermediatepressure.

If there are multiple compressor stages, then for one stage one can usethe flash intercooler and for another one can use a thermal storage heatexchanger. However, it is possible to use flash intercoolersexclusively, for example, two flash intercoolers. In this case, theremay not be any reheat stages in the discharging cycle. Further, it ispossible to use thermal storage heat exchangers for intercoolingexclusively, for example, two heat exchangers. In this case there may bemore than one reheat stage in the discharging cycle.

It is noted that the charging cycle of a thermoelectric energy storagesystem may be referred to as a heat pump cycle and the discharging cycleof a thermoelectric energy storage system may be referred to as a heatengine cycle. In the thermoelectric energy storage concept, heat needsto be transferred from a hot working fluid to a thermal storage mediumduring the charging cycle and back from the thermal storage medium tothe working fluid during the discharging cycle. A heat pump requireswork to move thermal energy from a cold source to a warmer heat sink.Since the amount of energy deposited at the hot side, for example, thethermal storage medium part of a thermoelectric energy storage, isgreater than the compression work by an amount equal to the energy takenfrom the cold side, for example, the heat absorbed by the working fluidat the low pressure, a heat pump deposits more heat per work input tothe hot storage than resistive heating. The ratio of heat output to workinput is called a coefficient of performance (COP), and it is a valuelarger than one. In this way, the use of a heat pump will increase theround-trip efficiency of a thermoelectric energy storage system.

The charging cycle of a thermoelectric energy storage system may includea work recovering expander, an evaporator, a compressor and a heatexchanger, all connected in series by a working fluid circuit. Further,a cold storage tank and a hot storage tank, for example, containing afluid thermal storage medium may be coupled together via the heatexchanger. Whilst the working fluid passes through the evaporator, itabsorbs heat from the ambient or from a thermal bath and evaporates. Thedischarging cycle of a thermoelectric energy storage system may includea pump, a condenser, a turbine and a heat exchanger, all connected inseries by a working fluid circuit. Again, a cold storage tank and a hotstorage tank, for example, containing a fluid thermal storage medium maybe coupled together via the heat exchanger. Whilst the working fluidpasses through the condenser, it exchanges heat energy with the ambientor the thermal bath and condenses. The same thermal bath, such as ariver, a lake or a water-ice mixture pool, may be used in both thecharging and discharging cycles.

Exemplary embodiments of the present disclosure overcome the problem ofan excessive temperature rise in the working fluid during compression inthe charging cycle. This problem occurs where the ratio of the highestoperating pressure of a transcritical thermoelectric energy storagesystem to the evaporator pressure of the charging cycle is relativelygreat. Specifically, this excessive temperature rise is detrimental tothe completion of the compression process in a single stage unless theworking fluid is heated to an acceptably high temperature.

Thus, the skilled person will appreciate that the present disclosureprovides a thermoelectric energy storage system where the charging anddischarging cycles are designed to have corresponding compressorintercooling and reheat sections, respectively, with matching heat loadsand temperature levels, and where an intercooler may be used for coolingeach of the additional compression stages of the charging cycle. Suchintercoolers may be located at the corresponding compressor dischargesand are fed with partially expanded working fluid from the condenserexit, such that the heat of compression is absorbed by the process ofvaporizing the liquid part of the working fluid.

In accordance with an exemplary embodiment, the present disclosureprovides a multi-stage compression system in which the working fluid iscooled close to its saturation temperature as it is output from eachintermediate compression stage. The heat released from the working fluidduring the cooling is recovered and utilized to improve roundtripefficiency of the thermoelectric energy storage system.

An exemplary embodiment of the present disclosure provides a method forstoring electrical energy in a charging cycle and retrieving electricalenergy in a discharging cycle.

According to an exemplary embodiment of the present disclosure, in thecharging cycle, the method includes compressing the working fluid from alow pressure to an intermediate pressure for storing electrical energy(for example, for converting electrical energy into heat energy),cooling the working fluid at the intermediate pressure, compressing theworking fluid from the intermediate pressure to a high pressure forstoring electrical energy, and transferring heat from the working fluidat the high pressure to the thermal storage.

According to an exemplary embodiment of the present disclosure, in thedischarging cycle, the method includes transferring heat from thethermal storage to the working fluid at the high pressure, and expandingthe working fluid from the high pressure for generating electricalenergy.

It is to be understood that features of the method as described hereinmay be features of the system as described herein.

If technically possible but not explicitly mentioned, combinations ofembodiments of the present disclosure described herein may beembodiments of the method and the system.

FIGS. 1 a and 1 b show a simplified schematic diagram of athermoelectric energy storage system 10 according to an exemplaryembodiment of the present disclosure.

The charging cycle system 12 shown in FIG. 1 a includes a firstcompression stage with a compressor 14, a second compression stage witha compressor 16, and a third compression stage with a compressor 18. Thecharging cycle system 12 also includes a first expansion stage with anexpander 20 and a second expansion stage with an expansion valve 22. Aworking fluid circulates through all components of a working fluidcircuit 24 as indicated by the solid line with arrows.

Further, the charging cycle system 12 includes a stream splitter 26between the expander 20 and the expansion valve 22, a flash intercooler28 between the compressor 14 and the compressor 16 and a heat exchanger30 between the compressor 16 and the compressor 18.

At the high pressure side 32, the charging cycle system 12 includes aheat exchanger 34, and at the low pressure side 36, the charging cyclesystem 12 includes a heat exchanger 38.

In operation, the charging cycle system 12 performs a transcriticalcycle and the working fluid flows around the thermoelectric energystorage system 10 in the following manner.

In the first expansion stage, the working fluid enters the expander 20where the working fluid is expanded from a high pressure to a lower(intermediate) pressure. On exiting the expander 20, the working fluidstream is split in two streams by the stream splitter 26, with a firstportion of the working fluid flowing to the second expansion stage withexpansion valve 22 and a second portion passing directly to the flashintercooler 28.

After the second expansion stage, where the working fluid is expanded byexpansion valve 22 from the intermediate pressure to a low pressure, theworking fluid passes to the heat exchanger 38 where the working fluidabsorbs heat from the ambient or from a cold storage 40 and evaporates.For example, the heat exchanger 38 is a counter flow heat exchanger 38and a cold storage medium circulates from a first cold storage tank 42to a second cold storage tank 44 for exchanging heat with the workingfluid.

The vaporised working fluid is circulated to a first compression stagein which surplus electrical energy is utilized to compress and heat theworking fluid in a compressor 14 from the low pressure to theintermediate pressure. On exiting the compressor 14, this first portionof working fluid is mixed with the relatively cooler, second portion ofworking fluid in the flash intercooler 28.

The mixed working fluids pass to a second compression stage whichincludes the compressor 16. In the second compression stage, furthersurplus electrical energy is utilized to compress the working fluid fromthe intermediate pressure to a higher second intermediate pressure. Theworking fluid mass flow through the second compression stage is greaterthan the working fluid mass flow through the first compression stage.

Next, the working fluid passes through the heat exchanger 30 where it iscooled as heat energy is transferred from the working fluid to a thermalstorage medium from a further heat storage 46. For example, the heatexchanger 30 is a counter flow heat exchanger 30 and the storage mediumcirculates from a first storage tank 48 to a second storage tank 50 forexchanging heat with the working fluid.

The working fluid is then directed to a third compression stage where itpasses through the compressor 18 before entering the heat exchanger 34.In the third compression stage, again surplus electrical energy isdriving the compressor 18 for compressing the working fluid from thesecond intermediate pressure to the (higher) high pressure.

Again, in the heat exchanger 34 heat energy is transferred from theworking fluid into a thermal storage medium from a hot storage 52. Forexample, the heat exchanger 34 is a counter flow heat exchanger 34 andthe storage medium circulates from a first hot storage tank 54 to asecond hot storage tank 56 for exchanging heat with the working fluid.

Finally, the working fluid is again directed into the first expansionstage.

In the exemplary embodiment of FIG. 1 a, the flash intercooler 28 is aspray intercooler 28. In accordance with other exemplary embodiments,other types of flash intercoolers 28 may be used.

Further, it should be noted that additional compression and expansionstages may be added. However, It should also be noted that at least oneintercooler 28, 30 is required in the charging cycle 12 in order toachieve improved efficiency of the system 10. For example, there may beonly two compression stages with a first compressor and a secondcompressor and only the flash intercooler 28 or the heat exchanger 30 inbetween the two stages (in the second case only one expansion stage maybe needed).

In accordance with an exemplary embodiment, each compression stage maybe equipped with a flash intercooler 28, when reheat options are notconsidered in the discharging cycle. It should be noted that differentworking fluids may be used for the different cycles, as long as thetemperature levels for the heat load of the heat pump, the heat storageand the heat engine are chosen appropriately.

In an exemplary embodiment, in which the working fluid is carbon dioxideand the thermal storage medium is water, the charging cycle may operatein the temperature range of 5° C. and 120° C. The intercooling occurs ata temperature levels well distributed within this range.

Summarized, according to an exemplary embodiment, the system 10 includesa first expander 20 configured to, in the charging cycle, expand theworking fluid after the first heat exchanger 34 to the intermediatepressure. In the charging cycle, a first portion of the working fluid atthe intermediate pressure is input directly into the flash intercooler28.

According to an exemplary embodiment, the system 10 includes a secondexpander 22 configured to, in the charging cycle, expand a secondportion of the working fluid at the intermediate pressure to the lowpressure.

According to an exemplary embodiment, the system 10 includes a thirdheat exchanger 38 configured to, in the charging cycle, transfer heatfrom a third thermal storage 40 to the working fluid at low pressureand, in the discharging cycle, transfer heat from the working fluid atlow pressure to the third thermal storage.

According to an exemplary embodiment, the intercooler includes a flashintercooler 28 and a third heat exchanger 30, wherein, in the chargingcycle, the working fluid between the flash intercooler and the thirdheat exchanger is compressed from a first intermediate pressure to asecond intermediate pressure by a further compressor 16.

With respect to FIG. 1 b, the heat stored in the heat storages 40, 46and 52 is subsequently utilized in the discharging cycle system 56 shownin FIG. 1 b.

The working fluid in the discharging cycle system 58 coming from theheat exchanger 38 is pumped from the low pressure to the high pressureby pump 60. After that the working fluid is heated in the heat exchanger34 and enters a first turbine 62 for converting the heat into mechanicaland subsequently into electrical energy. The working fluid is reheatedagain in heat exchanger 30 and enters a second turbine 64 for generatingfurther electrical energy. In the first turbine 62 the working fluid isexpanded from the high pressure to the intermediate pressure and in thesecond turbine 64 to the low pressure. After that the working fluid iscooled in the heat exchanger 38.

According to an exemplary embodiment, the system 10 includes a firstturbine 62 configured to, in the discharging cycle, expand the workingfluid from the high pressure to the intermediate pressure for generatingelectrical energy and/or a second turbine 64 configured to, in thedischarging cycle, expand the working fluid from the intermediatepressure to the low pressure for generating electrical energy.

According to an exemplary embodiment, the system 10 includes a pump 60configured to, in the discharging cycle, pump the working fluid from thelow pressure to the high pressure during the discharging cycle.

FIGS. 2 a and 3 a show a charging cycle 12 a, 12 b, and FIGS. 2 b and 3b show a discharging cycle 58 a, 58 b of a transcritical thermoelectricenergy storage system 10. The cycles are depicted in pressure-enthalpydiagrams.

In each of the diagrams, a vapor dome 66 is indicated. The criticalpoint 68 of the working fluid is shown on top of the vapor dome. Left ofthe vapor dome 66, the working fluid is in liquid phase, and right ofthe vapor dome 66, the working fluid is in gas phase (wet steam phase).Under the vapor dome 66, the working fluid is in a mixed liquid and gasphase. A phase change of the working fluid only occurs, when a statechange passes the limiting line of the vapor dome 66. Thus, statedchange over the vapor dome 66 and over the critical point 68 do notcontain phase changes and may be called transcritical. As may be seenfrom the diagrams, nearly all state changes of the working fluid in thecharging cycle and the discharging cycle are transcritical, andtherefore the charging cycle and the discharging cycle are referred toas transcritical.

FIG. 2 a illustrates the charging cycle 12 of a storage system 10 whichmay include two heat exchangers 30 for intercooling the working fluid.The charging cycle 12 a follows a counter-clockwise direction asindicated by the arrows. The charging cycle 12 a starts at point A wherethe working fluid is first evaporated at a low pressure 70 by utilizing,for example a low grade heat source such as ambient air or by a heatexchanger 38. This transition is indicated in FIG. 2 a with the linefrom point A to point B1.

In the next section of the charging cycle 12 a, the resultant vapor iscompressed utilizing electrical energy in three stages from point B1 toC1 to a first intermediate pressure 72, from B2 to C2 to a secondintermediate pressure 74, and from B3 to C3 to a high pressure 76. Suchcompression occurring in three stages is a consequence of thethermoelectric energy storage 10 having a compressor train comprisingthree individual units, for example the compressors 14, 16, 18. Inbetween each of these compression stages the working fluid is cooledfrom point C1 to B2 and point C2 to B3. For example, the working fluidmay be cooled by two heat exchangers 30.

The hot compressed working fluid exiting the compression train at pointC3 is cooled down at constant pressure 76 to point D, for example in aheat exchanger 34. Since the cycle 12 a is supercritical between thepoints C3 and D, no condensation of the working fluid takes place. Theheat rejected between point C1 to B2, C2 to B3, and C3 and D istransferred to a thermal storage medium via heat exchangers 30, 34,thereby storing the heat energy. After reaching point D, the cooledworking fluid is returned to its initial low pressure state 70 at pointA via a thermostatic expansion valve 22 or alternatively with an energyrecovering expander.

FIG. 2 b illustrates the discharging cycle of a thermoelectric energystorage system 10 with one turbine 62 that follows a clockwise directionas indicated by the arrows. The discharging cycle 58 a starts with thecompression of the working fluid as it is pumped from point E to point Ffrom low pressure 70 to high pressure 76, for example by pump 60. Frompoint F to point G, the working fluid is in contact with the thermalstorage medium in a direct or indirect manner, wherein stored heat istransferred from the thermal storage medium to the working fluid. Forexample, this may be done with a heat exchanger 34. The working fluid isin a supercritical state between point F and point G, hence noevaporation takes place.

The subsequent expansion of the working fluid in a turbine 62 frompressure 76 to pressure 70 in order to generate electricity isrepresented between point G and point H. Finally, the working fluid iscondensed to its initial state by exchanging heat, for example with acooling medium such as ambient air or with a cold storage 40 via a heatexchanger 38. This is represented from point H to point E on FIG. 2 b.

When both thermodynamic cycles 12 a, 58 a shown in FIGS. 2 a and 2 bwould use the same working fluid, it is noted that the total heat energygenerated in the charging cycle 12 a is greater than the heat energyrequirement of the discharging cycle 58 a. Specifically, the total heatenergy required for functioning of the discharging cycle 58 a, which isequal to the enthalpy difference from point F to point G in FIG. 1 b,can be provided solely by the heat energy released during the chargingcycle between point C3 and point D in FIG. 1 a.

Therefore, it would be beneficial to efficiently utilize the excess heatresulting from compressor intercooling. However, this excess heat cannotbe used to increase the enthalpy content at point G (which may beenvisaged as pushing point G further to the right in the cycle in FIG. 1b), because the temperature at which this excess heat is available islower than the temperature of point G. Thus, according to an exemplaryembodiment of the present disclosure, a storage system 10 with acharging cycle 12 a includes a discharging cycle, wherein the heatstored during intercooling is used for reheating the working fluidbetween the expansions in turbines 62, 64.

Also, the excess heat generated by intercooling may not be used toincrease the power output of the discharging cycle 58 a throughincreasing the working fluid flow. Thus, according to an exemplaryembodiment of the present disclosure, a storage system 10 with adischarging cycle 58 a includes a discharging cycle, wherein a flashintercooler 28 is used for cooling the working fluid between twocompression stages.

FIG. 3 a and FIG. 3 b depict a charging cycle 12 b and a dischargingcycle 58 b, respectively, on a pressure-enthalpy diagram, which may beperformed by an exemplary embodiment of the transcritical thermoelectricenergy storage system 10 shown in FIGS. 1 a and 1 b.

Referring first to FIG. 3 a, the charging cycle 12 b follows acounter-clockwise direction as indicated by the arrows. The chargingcycle 12 b starts with the expansion of the working fluid which occursin two stages, between point D and point A1 from pressure 76 to pressure72 (expander 20), and between point A1 and point A2 from pressure 72 topressure 70 (expansion valve 22). The working fluid stream is divided atpoint A1 (stream splitter 26), where a first portion is diverted topoint B2 and the remaining portion is expanded further to point A2(expansion valve 22).

There is an increase in enthalpy in the remaining portion as it reachespoint B1 and in the first compression stage between B1 and C1 frompressure 70 to pressure 72 (compressor 14) and there is an increase inboth pressure and enthalpy. The discharge of this first compressionstage is cooled by intercooling (intercooler 28). Specifically, point B2represents the flash intercooler 28 where the hot working fluid frompoint C1 is mixed with the expanded working fluid from point A1.

The discharge from the second compression stage, between point B2 andpoint C2 from pressure 72 to pressure 74, is directed to a heatexchanger 30 where the thermal energy of the working fluid is deliveredto a thermal energy storage 46 between points C2 and B3.

The third compression stage from pressure 74 to pressure 76 occursbetween points B3 and C3 (compressor 18).

Such compression occurring in three stages is a consequence of thethermoelectric energy storage 10 having a compressor train comprisingthree individual units 14, 16, 18. In between each of these compressionstages the working fluid is cooled from point C1 to B2 and point C2 toB3 at constant pressure.

Similarly, the hot compressed working fluid exiting the compressiontrain at point C3 is cooled down at constant pressure 76 to point D(heat exchanger 34). Since the cycle 12 b is supercritical between thepoints C3 and D, no condensation of the working fluid takes place. Therejected heat energy between points C3 and D is stored in a thermalstorage medium (hot storage 52). After reaching point D, the cooledworking fluid is returned to its initial low pressure state 70 at pointA1 via a work recovering expander 20/thermostatic expansion valve 22.

The flash intercooler 28 utilized in the charging cycle 12 b may be adirect-contact heat exchanger, where the liquid working fluid from pointA1 to be evaporated is injected or sprayed into the compressed workingfluid vapour flow at C1. Such a direct-contact heat exchanger includes ashell filled with a packing of a high specific surface area in order toincrease the wetted heat transfer area.

FIG. 3 b illustrates the discharging cycle 58 b of the thermoelectricenergy storage system 10 that follows a clockwise direction as indicatedby the arrows. The discharging cycle starts with the compression (pump60) of the working fluid from low pressure 70 to high pressure 76 andthis transition is indicated in FIG. 3 b with the line from point E topoint F.

From point F to point G1, the working fluid is in contact with thethermal storage medium in a direct or indirect manner, wherein storedheat is transferred from the thermal storage medium to the working fluidat constant pressure (heat exchanger 34). The working fluid is in asupercritical state between point F and point G1, hence no evaporationtakes place.

The subsequent expansion of the working fluid in a turbine 62 in orderto generate electricity is represented between point G1 and point H1.Between points H1 and G2 there is a reheat stage at pressure 74, wherethe reheat energy is provided from the thermal storage 46. Specifically,the thermal storage 46 is coupled to the heat exchanger 30 correspondingto the second intercooling stage in the charging cycle 12 b.

The second expansion of the working fluid from G2 to H2 from pressure 74to pressure 70 occurs in a second turbine stage (turbine 64). Finally,the working fluid is condensed to its initial state at constant pressureby exchanging heat with a cooling medium such as ambient air or with aheat exchanger 38. This is represented from point H2 to point E on FIG.3 b.

It should be noted that, in accordance with an exemplary embodiment inwhich reheat options are not utilized in the discharging cycle, thenevery compressor stage in the charging cycle can be equipped with aseparate flash intercooler.

In accordance with an exemplary embodiment, different working fluids maybe utilized in the charging and discharging cycles. However, thetemperature levels for the charging cycle, the heat storage and thedischarging cycle must be adjusted to ensure transfer of heat in thedesired direction.

In accordance with an exemplary embodiment, water is used as the workingfluid in the charging cycle. Furthermore, another fluid with a highboiling point may be utilized instead of water. In this embodiment, theintercooling heat load is at a suitably high temperature to be storedand used to drive a secondary discharging cycle having a low boilingpoint working fluid (such as hydrocarbon). In this embodiment, thermalenergy stored during intercooling can be efficiently recovered withoututilizing a flash intercooler.

The skilled person will be aware that the condenser and the evaporatorin the thermoelectric energy storage system may be replaced with amulti-purpose heat exchange device that can assume both roles, since theuse of the evaporator in the charging cycle and the use of the condenserin the discharging cycle will be carried out in different periods.Similarly the turbine and the compressor roles can be carried out by thesame machinery, referred to herein as a thermodynamic machine, capableof achieving both tasks.

Further the temperatures, the pressures and the amount of working fluidexiting the stream splitter 26 may be measured and these values may becontrolled by valves situated in the working fluid circuit.

While the present disclosure has been illustrated and described indetail in the drawings and foregoing description, such illustration anddescription are to be considered illustrative or exemplary and notrestrictive. The present disclosure is not limited to the disclosedembodiments. Other variations to the disclosed embodiments can beunderstood and effected by those skilled in the art and practicing theclaimed disclosure, from a study of the drawings, the presentdisclosure, and the appended claims. In the claims, the word“comprising” or “including” does not exclude other elements or steps,and the indefinite article “a” or “an” does not exclude a plurality. Asingle processor or controller or other unit may fulfill the functionsof several items recited in the claims. The mere fact that certainmeasures are recited in mutually different dependent claims does notindicate that a combination of these measures cannot be used toadvantage. Any reference signs in the claims should not be construed aslimiting the scope.

It will be appreciated by those skilled in the art that the presentinvention can be embodied in other specific forms without departing fromthe spirit or essential characteristics thereof. The presently disclosedembodiments are therefore considered in all respects to be illustrativeand not restricted. The scope of the invention is indicated by theappended claims rather than the foregoing description and all changesthat come within the meaning and range and equivalence thereof areintended to be embraced therein.

What is claimed is:
 1. A thermoelectric energy storage system forstoring electrical energy by transferring thermal energy to a thermalstorage in a charging cycle, and for generating electricity byretrieving the thermal energy from the thermal storage in a dischargingcycle, the thermoelectric energy storage system comprising: a workingfluid circuit configured to circulate a working fluid; a firstcompressor configured to, in the charging cycle, compress the workingfluid from a low pressure to an intermediate pressure; an intercoolerconfigured to, in the charging cycle, cool the working fluid at theintermediate pressure; a second compressor configured to, in thecharging cycle, compress the working fluid from the intermediatepressure to a high pressure; and a first heat exchanger configured to,in the charging cycle, transfer heat from the working fluid at the highpressure to the thermal storage and, in the discharging cycle, transferheat from the thermal storage to the working fluid at the high pressure.2. The system according to claim 1, wherein the intercooler includes aflash intercooler.
 3. The system according to claim 1, wherein theintercooler includes a second heat exchanger configured to, in thecharging cycle, transfer heat from the working fluid at the intermediatepressure to a second thermal storage and, in the discharging cycle,transfer heat from the second thermal storage to the working fluid atintermediate pressure.
 4. The system according to claim 1, comprising: afirst expander configured to, in the charging cycle, expand the workingfluid after the first heat exchanger to the intermediate pressure,wherein, in the charging cycle, a first portion of the working fluid atthe intermediate pressure is input into the intercooler.
 5. The systemaccording to claim 4, comprising: a second expander configured to, inthe charging cycle, expand the working fluid at the intermediatepressure to the low pressure.
 6. The system according to claim 1,comprising: a third heat exchanger configured to, in the charging cycle,transfer heat from a third thermal storage to the working fluid at thelow pressure and, in the discharging cycle, transfer heat from theworking fluid at the low pressure to the third thermal storage.
 7. Thesystem according to claim 1, wherein: the intercooler includes a flashintercooler and a third heat exchanger; and in the charging cycle, theworking fluid between the flash intercooler and the third heat exchangeris compressed from a first intermediate pressure to a secondintermediate pressure.
 8. The system according to claim 1, comprising: afirst turbine configured to, in the discharging cycle, expand theworking fluid from the high pressure to the intermediate pressure forgenerating electrical energy; and a second turbine configured to, in thedischarging cycle, expand the working fluid from the intermediatepressure to the low pressure for generating electrical energy.
 9. Thesystem according to claim 1, comprising: a pump configured to, in thedischarging cycle, pump the working fluid from the low pressure to thehigh pressure during the discharging cycle.
 10. A method for storingelectrical energy in a charging cycle and retrieving electrical energyin a discharging cycle, wherein, in the charging cycle, the methodcomprises: compressing the working fluid from a low pressure to anintermediate pressure for storing electrical energy; cooling the workingfluid at the intermediate pressure; compressing the working fluid fromthe intermediate pressure to a high pressure for storing electricalenergy; and transferring heat from the working fluid at the highpressure to the thermal storage, and wherein, in the discharging cycle,the method comprises: transferring heat from the thermal storage to theworking fluid at the high pressure; and expanding the working fluid fromthe high pressure for generating electrical energy.
 11. The methodaccording to claim 10, wherein: in the charging cycle, the methodcomprises transferring heat from the working fluid at the intermediatepressure to a second thermal storage; and in the discharging cycle, themethod comprises: expanding the working fluid from the high pressure tothe intermediate pressure for generating electrical energy in a firstturbine; transferring heat from the second thermal storage to theworking fluid at the intermediate pressure; expanding the working fluidfrom the intermediate pressure to the low pressure for generatingelectrical energy in a second turbine.
 12. The method according to claim10, wherein, in the charging cycle, the method comprises: expanding theworking fluid after the heat exchanging at the high pressure to theintermediate pressure; and using a first portion of the working fluid atintermediate pressure after the heat exchanging at the high pressure forcooling the working fluid before heat exchanging at the high pressure.13. The method according to claim 10, wherein, in the charging cycle,the method comprises: expanding the working fluid at the intermediatepressure to the low pressure; and transferring heat from a third thermalstorage to the working fluid at the low pressure, and wherein, in thedischarging cycle, the method comprises transferring heat from theworking fluid at the low pressure to the third thermal storage.
 14. Themethod according to claim 10, wherein, in the charging cycle, the methodcomprises compressing the working fluid from a first intermediatepressure to a second intermediate pressure between a flash intercoolingwith the working fluid at the first intermediate pressure and heatexchanging with a second thermal storage at the second intermediatepressure.
 15. The method according to claim 10, wherein at least onesection of at least one of the charging cycle and the discharging cycleis performed transcritically.
 16. The system according to claim 2,wherein the intercooler includes a second heat exchanger configured to,in the charging cycle, transfer heat from the working fluid at theintermediate pressure to a second thermal storage and, in thedischarging cycle, transfer heat from the second thermal storage to theworking fluid at intermediate pressure.
 17. The system according toclaim 16, comprising: a first expander configured to, in the chargingcycle, expand the working fluid after the first heat exchanger to theintermediate pressure, wherein, in the charging cycle, a first portionof the working fluid at the intermediate pressure is input into theintercooler.
 18. The system according to claim 17, comprising: a secondexpander configured to, in the charging cycle, expand the working fluidat the intermediate pressure to the low pressure.
 19. The systemaccording to claim 16, comprising: a third heat exchanger configured to,in the charging cycle, transfer heat from a third thermal storage to theworking fluid at the low pressure and, in the discharging cycle,transfer heat from the working fluid at the low pressure to the thirdthermal storage.
 20. The system according to claim 16, wherein: theintercooler includes a flash intercooler and a third heat exchanger; andin the charging cycle, the working fluid between the flash intercoolerand the third heat exchanger is compressed from a first intermediatepressure to a second intermediate pressure.
 21. The system according toclaim 16, comprising: a first turbine configured to, in the dischargingcycle, expand the working fluid from the high pressure to theintermediate pressure for generating electrical energy; and a secondturbine configured to, in the discharging cycle, expand the workingfluid from the intermediate pressure to the low pressure for generatingelectrical energy.
 22. The system according to claim 16, comprising: apump configured to, in the discharging cycle, pump the working fluidfrom the low pressure to the high pressure during the discharging cycle.23. The method according to claim 10, wherein the compressing of theworking fluid from the low pressure to the intermediate pressureincludes storing the electrical energy for converting the electricalenergy into heat energy.
 24. The method according to claim 11, wherein,in the charging cycle, the method comprises: expanding the working fluidafter the heat exchanging at the high pressure to the intermediatepressure; and using a first portion of the working fluid at intermediatepressure after the heat exchanging at the high pressure for cooling theworking fluid before heat exchanging at the high pressure.
 25. Themethod according to claim 24, wherein, in the charging cycle, the methodcomprises: expanding the working fluid at the intermediate pressure tothe low pressure; and transferring heat from a third thermal storage tothe working fluid at the low pressure, and wherein, in the dischargingcycle, the method comprises transferring heat from the working fluid atthe low pressure to the third thermal storage.
 26. The method accordingto claim 25, wherein, in the charging cycle, the method comprisescompressing the working fluid from a first intermediate pressure to asecond intermediate pressure between a flash intercooling with theworking fluid at the first intermediate pressure and heat exchangingwith a second thermal storage at the second intermediate pressure. 27.The method according to claim 25, wherein at least one section of atleast one of the charging cycle and the discharging cycle is performedtranscritically.